Adverbs are the chameleons of grammar. They usually modify verbs, but they can also modify adjectives, other adverbs, phrases, and even entire sentences. They are magical, and they are, of course, categorized.
Now let's roll.
Adverbs of time answer the questions when?, for how long?, and how often?. They are very common:
It rained yesterday.
I'll call soon.
Nothing lasts forever.
I rarely forget my keys.
They stayed late.
The change is now under consideration.
Here are a few more:
Nowadays it's different.
I work nights.
They travel weekends.
There's something distinctive about that second group: they end in s. In fact, they look like plural nouns being used as adverbs. Their historical provenance becomes clearer if you imagine an apostrophe before that s.
The history of adverbs like nowadays starts with the genitive. That's the noun or pronoun form used to show that someone or something owns, controls, or is associated with someone or something else. In modern English, the genitive is usually shown with 's, as in "doing the day's work."
In Old English, the genitive was shown with es. And the genitive of some nouns could be used adverbially. For example, the genitive of the Old English word for day could be used to mean "in the daytime repeatedly."
This is how modern English came to have -s as a suffix used to form adverbs that denote usual or repeated action or state, as in nowadays et al.